This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-157061, filed May 26, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a color cathode ray tube provided with a shadow mask.
In general, a color cathode ray tube comprises a vacuum envelope that includes a panel and a funnel. The panel includes a substantially rectangular effective portion having curved inner and outer surfaces and a skirt portion that is set up on the periphery of the effective portion. The funnel is bonded to the skirt portion. A substantially rectangular phosphor screen is provided on the inner surface of the panel effective portion. The phosphor screen includes black non-luminous layers and three-color phosphor layers that are embedded in gaps between the non-luminous layers and glow blue, green, and red, individually. Further, a substantially rectangular shadow mask is opposed to the phosphor screen. On the other hand, an electron gun for accelerating, focusing, and emitting three electron beams is located in a neck of the funnel.
In the color cathode ray tube constructed in this manner, the three electron beams emitted from the electron gun are deflected by means of magnetic fields, that are generated by a deflecting yoke mounted on the outside of the funnel, so as to horizontally and vertically scan the phosphor screen through the shadow mask, whereupon a color image is displayed.
The shadow mask is provided with a mask body and a mask frame. The mask body includes a substantially rectangular principal mask surface that is formed of a curved surface opposed to the phosphor screen and a skirt portion that is formed by bending the peripheral edge portion of the principal surface. A large number of electron beam passage apertures are formed in the principal mask surface and arranged at given pitches. The mask frame is welded to the skirt portion of the mask body. The electron beam passage apertures of the mask body serve to screen the electron beams that are landed on the three-color phosphor layers.
In order to display an image free from color drift on the phosphor screen, the electron beam passage apertures of the mask body and their corresponding phosphor layers must be kept in specific relative positions. During the operation of the color cathode ray tube, the positional relations between the electron beam passage apertures and the phosphor layers, especially the distance (value q) between the principal mask surface of the mask body and the inner surface of the panel, must be kept within a given allowable range.
On the other hand, the electron beams that pass through their corresponding electron beam passage apertures and reach the phosphor screen during the operation of the color cathode ray tube account for ⅓ or less of all the electron beams that are emitted from the electron gun. The remaining electron beams run against some other portions of the mask body than the electron beam passage apertures and are converted into thermal energy, thereby heating the mask body. As the mask body is heated in this manner, it undergoes thermal expansion, whereupon so-called doming occurs such that the mask body bulges toward the phosphor screen. If this doming causes the distance between the principal mask surface of the mask body and the inner surface of the panel to exceed the given allowable range, the positions in which the electron beams are landed on the phosphor layers shift, so that the color purity lowers. This shift of landing of the electron beams on the phosphor layers substantially varies depending on the luminance or duration of image patterns.
As modern color cathode ray tubes become larger in size, their deflection angles are widened, while their screens are made flatter. In the color cathode ray tubes of this type, the color purity is lowered more drastically by the thermal expansion of the mask body. In many of flat-screen wide-type color cathode ray tubes, Invar (36%xe2x80x94Nixe2x80x94Fe alloy plate) with a low coefficient of thermal expansion is used for their mask body. In order to avoid an increase in cost of the shadow mask, in this case, a cold-rolled steel plate that is less expensive than Invar is frequently used for the mask frame.
If the mask body and the mask frame are formed of Invar and the cold-rolled steel plate, respectively, however, heat of the mask body that is generated by prolonged operation of the color cathode ray tube is transmitted to the mask frame, so that the mask frame undergoes thermal expansion. Owing to the difference in thermal expansion between the low-expansion mask body and the high-expansion mask frame causes the mask body, in this case, the mask body is pulled and deformed by the mask frame. In a manufacturing process for the color cathode ray tube, moreover, the shadow mask is heated to a higher temperature than by heating that is attributable to the electron beam collision. Accordingly, the difference in thermal expansion between the mask body and the mask frame increases, so that the mask body is deformed considerably.
In order to reduce the deformation of the mask body that is attributable to the difference in thermal expansion between the mask body and the mask frame, there is provided a shadow mask designed so that the skirt portion of the mask body is located inside the mask frame, and tongue portions on the skirt portion are welded to the mask frame. If the mask frame pulls the mask body, with this arrangement, the tongue portions are elastically deformed to absorb the pulling force of the mask frame, thereby reducing the deformation of the principal mask surface.
Although the shadow mask is constructed in this manner, however, it is inevitably large-sized if it is used in a large-sized color cathode ray tube. Thus, the mask body and the mask frame are subject to a substantial difference in elongation that is attributable to the difference in thermal expansion. The pulling force of the mask frame and the extent of the pull increase in proportion to the difference in elongation. In a color cathode ray tube with a flat screen, moreover, the curvature of the principal surface of the mask body is smaller, and its curvature retention strength is smaller than that of a conventional shadow mask. Accordingly, the principal mask surface is inevitably deformed even if the pulling force is small. In the shadow mask of this type, therefore, the pulling force that acts on the principal mask surface should be minimized.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a color cathode ray tube designed so that lowering of the color purity that is attributable to deformation and dislocation of a shadow mask is reduced to improve the image quality.
In order to restrain deformation that is attributable to the difference in thermal expansion between a mask body and a mask frame, it is to be desired that tongue portions of a skirt portion of the mask body should be shaped so that they are as deformable as possible. These tongue portions should be narrow and long.
In this case, however, the tongue portions are liable to be deformed in the crosswise direction, so that the mask body can be dislocated in a direction perpendicular to the tube axis inside the mask frame when it is subjected to a crosswise shock or vibration. In consequence, landing on a phosphor screen shifts, and the color purity lowers. In a phosphor screen forming process, moreover, phosphor layers are exposed with use of a shadow mask as a photo mask, so that attachment to and detachment of the shadow mask from a panel are repeated several times. These attachment and detachment operations cause the mask body to be dislocated, so that the resulting phosphor layers are dislocated, and a desired phosphor screen cannot be formed.
Recently, in particular, there has been an increasing demand for images of higher quality, and the arrangement pitches of the phosphor layers have been narrowed. Thus, the landing allowance for electron beams is so small that more accurate beam landing is required.
Accordingly, a color cathode ray tube according to the present invention comprises: an envelope having a panel including a substantially rectangular effective portion, a tube axis, a long axis perpendicular to the tube axis, and a short axis extending at right angles to the tube axis and the long axis; a phosphor screen formed on the inner surface of the effective portion and including a plurality of phosphor layers; a shadow mask opposed to the phosphor screen in the envelope; and an electron gun located in the envelope, for emitting electron beams to the phosphor screen through the shadow mask.
The shadow mask includes a substantially rectangular mask body having a substantially rectangular principal mask surface opposed to the phosphor screen and formed having a large number of electron beam passage apertures, a skirt portion formed bent around the principal mask surface, and a long axis and a short axis corresponding to the aforesaid long and short axes, respectively, and a substantially rectangular mask frame attached to the skirt portion and situated outside the skirt portion, the mask frame having a coefficient of thermal expansion higher than that of the mask body.
The skirt portion of the mask body includes a first tongue portion situated on the short axis and extending in the direction of the tube axis and a second tongue portion situated on the long axis and extending in the direction of the tube axis, the first and second tongue portions each having a free end portion and being fixed to the mask frame. The mask body is formed so as to fulfill relations given by
(Caxc2x7Wb) less than (Cbxc2x7Wa), and V less than H,
where Ca is the length of the first tongue portion in the direction of the tube axis, Wa is the width of the first tongue portion, Cb is the length of the second tongue portion in the direction of the tube axis, Wb is the width of the second tongue portion, H is the length of the principal mask surface in the direction of the long axis, and V is the length in the direction of the short axis.
According to the color cathode ray tube of the invention, moreover, the mask body is formed so as to fulfill a relation given by
(Caxc2x7Wb)/(Cbxc2x7Wa)xe2x89xa6V/H.
According to the color cathode ray tube of the invention constructed in this manner, the mask body with the low coefficient of thermal expansion is pulled by means of the mask frame with the high coefficient of thermal expansion if the shadow mask is heated. Since the first and second tongue portions on the short and long axes of the skirt portion of the mask body are individually welded to the mask frame, in this case, they are elastically deformed outward when the mask body is pulled by means of the mask frame owing to the difference in thermal expansion between the mask body and the mask frame. Thus, the pulling force that acts on the mask body and the extent of the pull can be absorbed or eased.
The pulling force that acts on the mask body as a result of thermal expansion of the mask frame and the extent of the pull are higher at the long axis end that is more distant from the center of the shadow mask than at the short axis end. Since the second tongue portion on the long axis of the mask body can be elastically deformed more easily and more heavily than the first tongue portion on the short axis, however, it can effectively absorb the pulling force that acts in the direction of the long axis of the mask body and the extent of the pull.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.